Deck calculator

What is a deck calculator?

A deck calculator helps you estimate the materials needed to build a wooden deck before you buy anything. From the overall size of the deck and the size of a single decking board, it works out the deck area, how many boards you need (including a waste allowance for cuts and off-cuts), the total lineal length of decking to purchase, and the number of joists in the supporting frame.

Planning these quantities in advance avoids two common problems: running short of boards partway through the job, or over-ordering and paying for material you never use. Because the calculator accepts both metric and imperial units on every measurement, you can enter dimensions in whatever system your supplier quotes.

How does the calculator work?

You provide five measurements:

• Deck length and deck width — the footprint of the finished deck.
• Board width and board length — the coverage of a single decking board.
• Joist spacing — the on-centre distance between the joists that carry the boards (commonly about 0.4 m / 16 in).

From these, the calculator computes the deck area, the number of boards (rounded up, with roughly 10% extra for waste), the total lineal length of decking, and the number of joists. Results are shown in read-only fields whose unit can be switched between metric and imperial on demand.

Formulas

Deck area is length times width:

$A = L \times W$

The number of boards divides the deck area by the coverage of one board, adds about 10% for waste, and rounds up to the next whole board:

$N = \left\lceil \frac{A \times 1.1}{w \times \ell} \right\rceil$

where $w$ is the board width and $\ell$ is the board length. The total lineal length of decking is then:

$D = N \times \ell$

The number of joists spans the deck width at the chosen spacing $s$, plus one to close off the final bay:

$J = \left\lceil \frac{W}{s} \right\rceil + 1$

Worked example

Consider a deck that is 6 m long and 4 m wide, built from boards 0.14 m wide and 3.6 m long, with joists spaced 0.4 m on centre.

1. Deck area: $A = 6 \times 4 = 24 \ \text{m}^2$
2. Board coverage: one board covers $0.14 \times 3.6 = 0.504 \ \text{m}^2$
3. Boards needed: $N = \left\lceil \frac{24 \times 1.1}{0.504} \right\rceil = \left\lceil 52.38 \right\rceil = 53$
4. Total lineal decking: $D = 53 \times 3.6 = 190.8 \ \text{m}$
5. Joists needed: $J = \left\lceil \frac{4}{0.4} \right\rceil + 1 = 10 + 1 = 11$

So the project needs 53 boards, about 190.8 m of decking in total, and 11 joists.

Practical notes

• The 10% waste allowance is a general rule of thumb. For simple rectangular decks it is usually enough; for diagonal or herringbone board patterns, or decks with many angles, allow more.
• Joist spacing depends on the board thickness and the span rules in your local building code. Closer spacing (e.g. 0.3 m) makes a stiffer deck but uses more timber.
• The board-count formula assumes boards are laid across the deck without accounting for the small gaps left between them for drainage and expansion. Those gaps slightly reduce the number of boards, so treating the result as a safe upper estimate is sensible.
• For the volume and price of individual lumber pieces, pair this with the board foot calculator, and use the square footage calculator if you only need the area.